The present invention relates to a load-sensitively controllable braking pressure control unit having at least one control valve associated with a brake circuit and arranged in a housing, which is a attachable to a vehicle part. An actuating tappet of the control valve protrudes from the housing and a transmission lever, which is acted upon by a first spring arrangement, is adapted to be swivelled relative to the housing and bears against the actuating tappet. The spring arrangement is mounted at a vehicle part for articulated movement relative to the housing by way of a length-adjustable connecting arrangement, at the free end of the transmission lever, and a second spring arrangement clamped between the transmission lever and the actuating tappet of the valve.
In load-sensitive braking pressure control units, the change-over point, i.e. the amount of braking pressure at which the pressure control comes in, is variable and in dependency on vehicle load. For that purpose, the braking pressure control unit is provided with a transmission lever, which is adapted to be swivelled, against which the control piston of the valve of the braking pressure control unit abuts and at which one end of a spring arrangement is articulated. The other end of the spring arrangement is mounted at a vehicle part for articulated movement and the position of which is load-sensitively variable relative to the braking pressure control unit. When the vehicle load is changed, the force of the spring arrangement acting upon the transmission lever is correspondingly changed.
It is difficult and expensive to install such a braking pressure control unit in a loaded vehicle and, in particular, to connect the spring arrangement to the vehicle part and to adjust the spring arrangement such that the actual change-over pressure of the valve just reaches the value desired with respect to the ideal braking pressure characteristic curve.
In the German printed and published patent application 20 27 573, a braking pressure control unit of this general type is disclosed, wherein two spring arrangements connected to a transmission lever are provided. One spring arrangement serves as a control spring and is connected to a vehicle part, which changes its position relative to the braking pressure control unit in a load-sensitive manner, through the intermediary of a length-adjustable connecting arrangement, whereas the other spring arrangement serves as a compensating spring and bears against the actuating tappet of the control valve. When the control spring is loaded in order to transmit a control force, the compensating spring is compressed and the transmission lever can abut directly against the actuating tappet of the valve without the intermediary of the interposed compensating spring. In this known braking pressure control unit it is particularly difficult to connect the spring arrangement to the vehicle part and to adjust the spring arrangement such that the control force generated by the two spring arrangements arranged one behind the other corresponds exactly to the control force desired in view of the ideal braking pressure characteristic curve. Since this arrangement provides that the force exerted in the event of an expansion of the compensating spring offsets the effect of the control spring, a slight shift of the length-adjustable connecting arrangement results in a considerable change of the control force.